A MEMS galvanic switch comprises a first electrode arrangement that is present on a substrate and a movable element that overlies at least partially the first electrode arrangement. The movable element is movable towards the substrate between a first and a second position by application of an actuation voltage.
In the first position, the movable element is separated from the substrate by a gap. The movable element comprises a second electrode that faces the first electrode arrangement. In the second position (closed switch) first and second electrodes are in mechanical and electrical contact with each other.
Known MEMS switches of this type can use electrostatic actuation in which electrostatic forces resulting from actuation drive voltages cause the switch to close. An alternative type uses piezoelectric actuation, in which drive signals cause deformation of a piezoelectric beam. This invention relates particularly to electrostatic switches.
Electrostatic galvanic MEMS switches are promising devices. They usually have 4 terminals: signal input, signal output, and two actuation terminals, one of which usually is kept at ground potential. By varying the voltage on the other actuation terminal, an electrostatic force is generated which pulls the movable structure downward. If this voltage is high enough, one or more contact dimple electrodes will touch and will provide a galvanic connection between the two signal terminals.
FIGS. 1 and 2 show one possible design of MEMS galvanic switch designed in accordance with known design principles.
In FIG. 1, the cross hatched pattern is the bottom electrode layer. This defines the signal in electrode 10, the signal out electrode 12 and lower actuation electrode pads 14. As shown, the actuation electrode pads 14 are grounded.
A top electrode layer defines the movable contact element 16 as well as the second actuation electrode 18 to which a control signal (“DC act”) is applied.
The second actuation electrode 18 has a large area overlapping the ground actuation pads so that a large electrostatic force can be generated. However, because the top actuation electrode 18 and the movable contact element 16 are formed from the same layer, a space is provided around the movable contact element 16. Furthermore, overlap of the actuation electrodes and the signal lines is undesirable, as explained further below.
FIG. 2 shows the device in cross section taken through a vertical line in FIG. 1. The same components are given the same reference numbers. FIG. 2 additionally shows the substrate arrangement 2 and the gap 20 beneath the movable contact element 16.
The connection between the signal input and signal output electrodes is made by the movable contact electrode which has two contact dimples as shown in FIG. 2. Galvanic MEMS switches can achieve low resistances Ron of less then 0.5 Ohm when they are switched on, and high isolation with small parasitic capacitance when they are off (Coff<50 fF). Typical dimensions are 30 to 100 μm outer diameter of the actuation electrode 18.
The device is manufactured in well known manner, in which sacrificial etching defines the gap 20.
When scaling galvanic MEMS switches down to lower sizes two problems occur:
the area of the RF in and RF out signal lines becomes relatively large and therefore reduces the area available for the actuation electrodes; and
if there is overlap between the signal lines and the actuation electrodes a large RF voltage on the signal line can cause attractive forces on the movable membrane. This can lead to undesired closing or prevent desired opening of the device. Moreover it can cause electrostatic discharges between the signal and actuation electrodes. In FIG. 1, only small connecting bars 22 of the actuation electrode 18 cross the signal lines; these provide structural rigidity to the suspended actuation electrode.
There is therefore a need for a design which enables sizes or actuation voltages to be reduced by maintaining strong electrostatic closing force and avoids interferences between conductor lines within the switch.